This invention relates to a method of and a device for measuring friction and wear under surrounding high pressure, where balls or rollers are pressed against each other by the measured force of an additional hydraulic unit inside a closed high-pressure autoclave, the torsional moment necessary to maintain the rotary motion is measured inside the autoclave, and in limiting cases the load is determined under which a sudden seizing and welding of the parts occurs.
The extraordinary financial losses yearly produced by friction and wear and the resulting machine damages early led to the development of test machines for determining the oil-quality, the maximum loading capacity of a machine, and the danger of machine damages by laboratory tests. Such test machines are for example: The Four Ball Testing Apparatus, the machines from Timken and Almen-Wieland, The Bartel-Lubrimeter, the Niemann-Gear-Rig (FZG-method), and the new two-disc test stand from Stobel and Niemann.
The often deversified valuation of an oil or lubricant resulting from comparative investigations with different test machines may be an indication that the lubricant should be in the best way adapted to the mechanical design and the specific plant conditions of the machine. Owing to the plurality of the entering parameters (for example geometry of the point of lubrication, surface quality of the machine parts, operating temperature, rotational speed, nominal value of the viscosity, temperature- and pressure-dependence of the viscosity) there is no possibility to constitute a simple scale of quality for lubricants. The lubricant has to be adapted to the considered machine and its characteristic working conditions.
The classical lubrication theory developed by Reynolds, Couette, Sommerfeld, and others already had to discuss the great difficulties, which arise in the theoretical treating of the phenomena of machine lubrication. With several omissions it was possible to develop formulas for the lubricating film-thickness and the coefficient of friction, which for lower machine loads correspond with experimental values and which can be used for the design of the geometrically simple and relative lowly loaded journal bearing.
But the progressive technical evolution led to special gearings showing very high contact pressures. Under these extreme stresses the classical lubrication theory delivers values for lubricating film thickness and coefficient of friction which are about 2 or 3 decimal powers too small. According to this theory mixed friction, wear, seizing, and welding of the machine parts are expected with increasing load, while multifarious experiments still show hydrodynamic lubrication.
As has become known in the meantime the reasons for the failure of the classical theory are the neglecting of the pressure dependence of the lubricant viscosity and the elastic deformation of machine parts under the very high pressure existing at the point of lubrication. These effects are considered by the modern elasto-hydrodynamic theory of lubrication (EHD-theory). Therewith the great discrepancies between theory and experiment essentially could be eliminated. But in more complicated cases and when the load further increases there are discrepancies and faulty expects in the EHD-theory, too.
It is known for certain that, considering the very complicated pressure- and temperature-shape at the point of lubrication of a machine, the pressure-dependence of the viscosity essentially influences friction and wear. With usual test machines the effects of the separate limiting quantities (pressure- and temperature-dependence of viscosity, compressibility in the whole pressure-temperature-range, geometry of the point of lubrication etc.) cannot be determined in detail. The test result comes about under superposition of several influences.